1. Field of the Invention
The present invention relates to a burner for synthesizing glass particles and a method for producing porous glass body in order to manufacture glass articles by a vapor phase synthesis method.
2. Description of the Related Art
As a method for obtaining a porous glass body which is a precursor for manufacturing various glass articles such as optical fibers, photo-mask materials, quartz glass, etc., there is generally used a vapor phase synthesis method such as a VAD (Vapor Phase Axial Deposition) method, an OVD (Outside Vapor Deposition) method, or the like. In such a method, raw material gas (SiCl4 or the like), combustible gas (hydrogen, methane, propane, or the like), and combustion-support gas (oxygen, or the like) are jetted from a plurality of different ports and mixed with one another to thereby form flames. The glass raw material is subjected to an oxidation reaction or a hydrolytic reaction in the flames formed thus so as to form glass particles flow. Then, the glass particles are deposited sequentially at a front end of a starting rod or around the starting rod.
Such a vapor phase synthesis reaction progresses in an area which is somewhat away from the outlets of the respective ports and in which the respective gases are mixed with one another. FIGS. 3A and 3B schematically show an example of the state of a reaction with which raw material is hydrolyzed or oxidized in a burner. In this example, a burner 16 includes gas jet ports formed in four layers concentrically, and glass raw material/combustible gas 9 is fed to a center port of the burner 16 while combustible gas 10, seal gas 11 and oxygen gas 12 are fed outside the center port. Thus, flames are formed to make a reaction. In a reaction area 13 in FIG. 3A, a hydrolytic reaction or oxidation reaction progress in accordance with the following formula. That is, in such a vapor phase synthesis method, effective mixture of the raw material and so on is a necessary condition for the improvement of the particles glass production efficiency, hence the improvement of the porous glass body deposition efficiency and the improvement of the raw material yield. Incidentally, in FIG. 3A, the reference numeral 14 represents a raw material gas/combustible gas development area, and 15, an oxygen gas development area.SiCl4+2H2+O2→SiO2+4HCl
As the burner for synthesizing glass particles in such a vapor phase synthesis method, there are known a concentric multi-tubular burner (Japanese Patent Unexamined Publication No. Sho. 61-183140, Japanese Patent Unexamined Publication No. Sho. 63-55135, etc.) in which a plurality of coaxial cyclic ports are disposed concentrically, a multi-nozzle burner (Japanese Patent Unexamined Publication No. Sho. 62-187135, Japanese Patent Unexamined Publication No. Hei. 6-247722, etc.) in which either combustion-support gas or combustible gas is fed from a plurality of individual nozzles, and so on.
In the concentric multi-tubular burner, raw material gas, combustion-support gas and combustible gas are jetted from coaxial cyclic ports disposed concentrically and mixed with one another to make the raw material undergo a hydrolytic reaction or an oxidation reaction in oxyhydrogen flames. Although the concentric multi-tubular burner has a merit to be manufactured easily, the jet directions of the raw material gas, the combustible gas (combustion gas) and the oxidizing gas (combustion-support gas) from opening end of the burner are substantially parallel with one another so that it cannot be said that the efficiency of mixing these gases is good. Therefore, it cannot be also said that the efficiency of the hydrolytic reaction or the oxidation reaction from the raw material to the glass particles is sufficiently high. Thus, there is a problem that the raw material yield (the ratio of the produced quantity of the porous glass body to the input quantity of the raw material) is comparatively low.
On the other hand, in the multi-nozzle burner, combustion-support gas or combustible gas is jetted from individual small-diameter nozzles so that raw material gas, combustion-support gas and combustible gas are mixed with one another to make the raw material undergo a hydrolytic reaction or an oxidation reaction in oxyhydrogen flames. The efficiency of gas mixture is high so that the efficiency of the vapor phase synthesis reaction increases and the raw material yield is improved. In addition, the flow rate of the gas jetted from each small-diameter nozzle is higher than that of another gas on the periphery thereof, and the flames are stable because of an excellent directivity, a small flow volume, and soon. However, in the multi-nozzle burner, a large number of small-diameter nozzles have to be disposed in a limited area of the burner. Thus, it takes a lot of trouble to manufacture the multi-nozzle burner in comparison with the concentric multi-tubular burner. In addition, the positions and directions of the nozzles affect the reaction extremely delicately, so that the raw material yield varies largely due to a slight displacement in the burner structure (nozzle layout or the like). Thus, there is a problem that it is difficult to obtain a stable burner performance and hence a stable porous glass body producing capacity.
As described above, in the multi-nozzle burner, oxygen or the like can be jetted toward the center portion (toward the raw material) from the small-diameter nozzles having directivity. Thus, the efficiency of mixture among raw material gas, combustion-support gas and combustible gas is high so that porous glass can be deposited with a high yield. On the contrary, there is a problem that the form of the nozzles largely affects the efficiency of deposition of the porous glass (there is a large individual difference among burners). In fact, it takes a lot of trouble to manufacture a large number of nozzles.
On the other hand, it is much easier to manufacture (to control the manufacturing of) the concentric multi-tubular burner than the multi-nozzle burner. However, a problem inheres in the concentric multi-tubular burner so that the efficiency of mixture among raw material gas, combustion-support gas and combustible gas is not as high as that conducted in the multi-nozzle burner. As a result of investigation of this reason, it was proved that the oxygen quantity diffusing to the center portion of the raw material was not always sufficient (in an oxygen diffusion rate-determining process). However, it was proved that if the supply oxygen quantity was increased, the temperature of the flames fell so that the efficiency of reaction of the raw material also dropped. It was therefore proved that simple increase in oxygen quantity could not solve the problem.
As a method for controlling refractive index of the optical fiber transmission area, there is a method for adding a proper quantity of GeO2 to SiO2 which is a main component of optical fibers. To add GeO2 to SiO2, there is generally used a method for supplying SiCl4 and GeCl4 simultaneously as raw material gas, more specially, for applying a proper quantity of GeCl4 which is a raw material of GeO2, as well as SiCl4 which is a raw material of SiO2, to the burner to deposit glass particles on a predetermined area when porous glass body which is a precursor for optical fiber. A glass particles synthesizing burner and a porous glass body producing method by use of the burner according to the present invention is effectively available when such mixed gas is used as raw material gas.
Oxygen diffusing and developing in the vicinity of the raw material flow is mainly consumed by the hydrolytic reaction of SiCl4 because the equilibrium constant of the hydrolytic reaction of GeCl4 is much smaller than that of SiCl4. Accordingly, in the case where SiCl4 and GeCl4 are mixed and fed into a raw material port, the hydrolytic reaction of SiCl4 progresses dominantly over the hydrolytic reaction of GeCl4, and thus reaction efficiency of GeCl4 is lowered. Therefore, to obtain a desired density of GeO2, it is necessary to apply excessive GeCl4.
According to a glass particles synthesizing burner and a porous glass body producing method by use of the burner of the present invention, oxygen closest to raw material is diffused toward the center portion so effectively that a larger amount of oxygen can be fed to the raw material development area. Thus, not only the hydrolytic reaction of SiCl4 but also the hydrolytic reaction of GeCl4 are accelerated remarkably. As a result, the reaction efficiency of SiCl4 and GeCl4, particularly, that of GeCl4, increases remarkably while the deposition speed of glass particles is enhanced.